Cathodic sputtering of films of stoichiometric zinc oxide



United States Patent 3,420,763 CATHODIC SPUTTERING 0F FILMS 0F STOICHIOMETRIC ZINC OXIDE William J. Polito and George A. Rozgonyi, Irvington,

N.J., assignors to Bell Telephone Laboratories, Incorporated, Murray Hill, Berkeley Heights, N.J., a corporation of New York No Drawing. Filed May 6, 1966, Ser. No. 548,044

US. Cl. 204-192 1 Claim Int. Cl. C23c 15/00 This invention relates to cathodic sputtering methods for obtaining thin films of zinc oxide.

Zinc oxide is a very promising piezoelectric material which has found few device applications due largely to the difliculty in producing the material in crystals of attractive size and quality.

For high frequency ultrasonic transducers thin films of a fraction to a few microns are required which suggests the growth of the crystal body by one of the several known thin film techniques. Efforts to evaporate zinc oxide have been unsuccessful. Reactive sputtering has been attempted in which a zinc cathode is sputtered in oxygen but the films produced are of poor quality for ultrasonic transducers. Post deposition oxidation of zinc films has been tried but this method is thickness limited and often produces a non-stoichiometric film unsuitable for high quality piezoelectric devices.

The method of this invention produces stoichiometric zinc oxide films of unexpectedly high quality and of any desired thickness over the range of principal interest (0.1 to 1.). It essentially involves sputtering from a zinc oxide cathode. It appears that the transport mechanism is such that the zinc oxide from the cathode is sputtered as an associated compound. This is important for the following several reasons.

The growth rate of the deposited film is dependent upon the vapor pressure of the molecular species being deposited. The vapor pressures of the constituents of zinc oxide, i.e., zinc and oxygen, are considerably higher than the reduced pressure required for sputtering. Thus even when the substrate is at a moderate temperature it is difiicult to retain an appreciable amount of zinc and oxygen on the substrate. Therefore, the growth rate of the deposited film is very low. For this reason sputtering a pure zinc cathode in a reactive atmosphere of oxygen according to ordinary procedures is not a satisfactory process.

The use of a zinc oxide cathode has been found to result in a deposition rate only slightly dependent on substrate temperature indicating a transfer of zinc oxide molecules which have a vapor pressure considerably lower than that of the constituent elements. This permits an appreciable growth rate even when the substrate is heated as high as 800 C. The crystal quality is improved by operating with higher substrate temperatures since the greater mobility thus realized permits the deposited molecules to rearrange into a highly ordered crystal habit.

The following specific embodiment is given by way of example.

The zinc oxide source material was fabricated into a cylindrical cathode by hot-pressing small single crystal filaments or whiskers of high purity zinc oxide into a mold 1.2 cm. in diameter and 3 cm. long. The vacuum system used was an all metal, sputter-ion pumped station with a water-cooled cylindrical reaction chamber 10 cm. in diameter and cm. long. The cathode-to-anode spac-- ing was 2 to 3 cm. although spacings of 1 cm. to 5 cm. are useful. The pressures used were of the order of 10p 3,420,763 Patented Jan. 7, 1969 ice to 300,44, although these can be extended to 1000;; with satisfactory results. These were established by evacuating the reaction chamber to 10 to 10* torr and backfilling with argon or other inert gas or oxygen or mixtures there- 5 of to reach the desired operating pressure. The cathode current density should be adjusted to within the range 0.1 to 10 ma./cm. Typical voltages to meet this requirement are from several hundred to a few thousand volts. Substrate temperatures may vary from 100 C. to 800 C. with the quality of the deposited film improving somewhat at the higher temperatures.

The resistivity of the deposited films may be varied by the choice of the atmosphere in which the sputtering is conducted. Typical resistivities are 0.03 ohm-cm. for films sputtered in argon and 10 ohm-cm. for films sputtered in pure oxygen. Intermediate values can be obtained by varying th oxygen/ argon pressure ratio.

Several samples of zinc oxide filmswere prepared in accordance with the invention by depositing zinc oxide films on single crystal sapphire and amorphous Pyrex glass slides and also on gold plated sapphire and quartz rods, the latter for evaluation as piezoelectric transducers. The specific conditions used in several runs were as follows. The cathode potential was 2000 volts giving a cathode current density of 2 ma./cm. Substrate temperatures were less than 600 C. The atmosphere was spectral grade argon maintained at to 100p. pressure. The deposition rates were 50 to 100 A./min. The films obtained were hexagonal zinc oxide oriented with the c-axis perpendicular to the substrate. Films were deposited in pure oxygen which increased the film resistivity, and with oxygen-argon mixtures. Whereas argon wasused in these experiments other inert gases such as krypton, helium or neon serve equally well.

Films sputtered in accordance with this invention were evaluated as piezoelectric transducers and showed insertion losses of 3.5 db per transducer for longitudinal mode at 400 mc. and 8.5 db for longitudinal mode at 1.2 gc. and 4 db at 200 mc. for shear mode. These values are unexpectedly low for transducers operating at these frequencies and under similar conditions and attest to the high resistivity and excellent crystal quality of the ZnO films.

What is claimed is:

1. A method for depositing a zinc oxide film which comprises sputtering from a cathode of zinc oxide onto a substrate placed within 1 cm. to 5 cm. of said cathode and maintained at a temperature in the range of 100 C. to 800 C. in a gaseous atmosphere selected from the group consisting of oxygen, an inert gas, and mixtures thereof at a cathode current density in the range of 0.1 to 10 ma./cm. and stopping said sputtering when said film has a thickness in the range of from about 0.1 micron to about 10 microns.

FOREIGN PATENTS 3/ 1960 Great Britain. 10/ 1958 Canada.

5/ 1962 Great Britain.

ROBERT K. MIHALEK, Primary Examiner.

US. Cl. X.R. 

1. A METHOD FOR DEPOSITING A ZINC OXIDE FILM WHICH COMPRISES SPUTTERING FROM A CATHODE OF ZINC OXIDE ONTO A SUBSTRATE PLACED WITHIN 1 CM. TO 5 CM. OF SAID CATHODE AND MAINTAINED AT A TEMPERATURE IN THE RANGE OF 100*C. AND 800*C. IN A GASEOUS ATMOSPHERE SELECTED FROM THE GROUP CONSISTING OF OXYGEN, AN INERT GAS, AND MIXTURES THEREOF AT A CATHODE CURRENT DENSITY IN THE RANGE OF 0.1 TO 10 MA./CM.2 AND STOPPING SAID SPUTTERING WHEN SAID FILM HAS A THICKNESS IN THE RANGE OF FROM ABOUT 0.1 MICRON TO ABOUT 10 MICRONS. 